In recent years, progress has been made in the microminiaturization of full duplex talking devices such as wireless hands-free earphone microphones, for example, and effects such as feedback or echo tend to be more likely to occur due to the reduction of the physical distance between the speakers and microphones used in earphone microphones. In addition, since it is essential that an earphone be placed in close proximity to the ear, it has become necessary to increase microphone sensitivity due to the increased distance between the microphone and the mouth producing the sound. This results in a situation in which it is extremely difficult to hear due to the effects of external noise or wind noise caused by high winds. In order to solve these problems, devices such as ultra-miniature hands-free earphone microphones incorporating echo prevention circuits or noise reduction circuits have been developed.
However, in a full duplex talking device in which the earphone and microphone are independent, in particular, since the microphone which generates transmission signals is placed in open space with constantly changing ambient acoustic characteristics, there are limits to the extent that factors inhibiting this communication can be predicted and reduced on a practical level. Even if circuits or algorithms necessary for echo prevention or noise reduction circuits are used, the scope of practical use is restricted by the effects of substantial external noise or wind noise due to strong wind.
Therefore, single-transducer type talking devices, which eliminate the effects of external noise by integrating the microphone inside the earphone, have been developed. For example, a full duplex talking circuit includes, a digital signal processing circuit is combined with an analog bridge circuit containing a single transducer to realize an echo canceling function which removes reception signals mixed with the transmission signals of the single transducer across almost all voice frequency bands.
However, various characteristics, including the strength of a transmission signal, generally change from moment to moment, and the transmission signal that is input into the single transducer is an audio signal that has passed through various organs of the body, including the eardrum. Therefore, it is necessary to compensate for the fact that the frequency characteristics have dramatically diminished due to the acoustic transfer characteristics of the human body.
In an actual situation in which the ambient acoustic environment changes in this way, not only does the echo canceling function fail to operate adequately in a conventional full duplex talking circuit, but transmission signals are also sometimes difficult to hear.
The present application provides a single-transducer full duplex talking circuit combining an analog bridge circuit and a digital signal processing circuit using a single transducer having a receiving and transmitting function. The transducer is isolated from the external environment to a certain extent and can therefore block external noise and wind noise which obstruct talking since. As described above, it is possible to sufficiently compensate for the deterioration of the frequency characteristics of the transmission signal wherein the echo canceling function operates adequately even in cases where various characteristics including the strength of a transmission signal change from moment to moment and in cases in which the frequency characteristics of a transmission signal that is input into the single transducer are dramatically diminished due to the acoustic transfer characteristics of the human body.